The present invention relates to a surface-treated steel sheet having a high corrosion resistance and a method for producing the same.
Conventionally, chromate treatment films have been widely used in primary anticorrosion treatment. The chromate treatment film is formed on a surface of a zinc-base-plated steel sheet to protect the surface from corrosion until a consumer uses the steel sheet. In recent years, however, even after a product has been fabricated using such a steel-sheet material, the steel-sheet material is still required to maintain the corrosion-resisting function.
Among zinc-base-plated steel sheets, a Znxe2x80x94Al-base-alloy-plated steel sheet has a relatively high corrosion resistance. The resistance is higher than that of the zinc-base-plated steel sheet. The Znxe2x80x94Al-base-alloy-plated steel sheet is therefore enjoying increasing demands in industrial fields, particularly in the field of building materials.
In the recent building-material field, however, the severity of requirements is increasing for the durability of the corrosion resistance and maintenance-free properties of materials. With this background, the appearance of the surface of the Znxe2x80x94Al-base-alloy-plated steel sheet is required to be durable for a longer period in various environments. Inherently, the appearance of the surface is required to be maintained in the fabrication of products in various shapes. As such, additional functions are required for the conventional chromate treatment film formed by applying a primary-rust-preventing treatment onto a Zn-5% Al-alloy-plated steel sheet that contains about 5 wt % Al to protect corrosion in a period until the steel sheet is used by a consumer. The required functions are as follows:
(a) a function (processed-portion corrosion resistance) of providing a high corrosion resistance even after the steel sheet fabricated into an intended product in a corrosive environment; and
(b) a function of inhibiting a blacken phenomenon in which the plated surface of the sheet material is blackened when the sheet material is stored outdoors for several days prior to fabrication.
Furthermore, the following functions are required for a Zn-55% Al plating alloy that contains about 55 wt % Al:
(a) a function of providing a high corrosion resistance even after the steel sheet fabricated into an intended product in a corrosive environment (processed-portion corrosion resistance); and
(b) a function of inhibiting a blacken phenomenon in which the plated surface of the sheet material is blackened in a humid environment (producing antiblackening resistance).
Chromate treatment films are broadly grouped into the following three types. They are an electrolysis-type chromate treatment film, a reaction-type chromate treatment film formed of a principal component of a trivalent-chromium compound, and a coating-type chromate treatment film formed of a compound of trivalent chromium and hexavalent chromium.
In these chromate-treatment films, the refractory trivalent chromium works as a barrier against corrosion-introducing factors, such as chloride ions and oxygen. That is, the refractory trivalent chromium provides barrier effects against the corrosion factors. On the other hand, in the coating-type chromate treatment film, the hexavalent chromium is dissolved out to a damaged portion of the chromate treatment film, and passivates the damaged portion. Thereby, the hexavalent chromium forms the film with corrosion-inhibiting effects (which hereinbelow will be referred to as xe2x80x9cself-healing effectsxe2x80x9d).
For the above-described reasons, the coating-type chromate treatment film is applied for coating in many cases in which the processed-portion corrosion resistance is required. However, since the hexavalent chromium has high oxidizeability, it is prone to be reduced to trivalent chromium as time passes. In addition, since the hexavalent chromium is water-soluble, it is prone to be dissolved out of the compositional system. Hence, in many cases, when the film is damaged, a phenomenon occurs in which the film loses residual hexavalent chromium sufficient to allow the film to impart self-healing effects. Thus, the provision of a sufficient processed-portion corrosion resistance cannot be insured.
In this field, there are known technical methods proposed to solve the above-described problems. The methods can be broadly grouped into two types (1) and (2) described as follows.
(1) Methods of a type for reducing the extent of damage on a film
For example, JP-A-2-34792, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies the xe2x80x9cunexamined Japanese patent publicationxe2x80x9d). discloses a method in which a fluorine-based resin is added in a chromate treatment film to have lubricity. Another example method of the type (1) is disclosed in JP-A-10-1762809. In this method, a thermoplastic elastomer is included in a film to impart ductility to the film. Concurrently, this method reduces the extent of a damaged film portion caused by sliding operation in, for example, press-forming. This enables the self-healing effects to be obtained with a relatively small amount of hexavalent chromium.
(2) Methods of a type for minimizing the dissolution amount of hexavalent chromium contained in a film
For example, one of the methods of the captioned type is disclosed in Domestic Republication of PCT International Publication for Patent Application No. 9-800337. According to the disclosed method, refractory chromium hydrochloric acid is dispersed within a film to inhibit chromium from being dissolved out in sound film portions. On the other hand, in a damaged film portion, corrosion reaction (the pH value increases) is used as a trigger to dissolve hexavalent chromium. This enables the film to impart self-healing effects.
In the methods of the type (1) above, a reduction can be achieved in regard to the extent of film damage caused in fabrication operations, such as press-forming and bending. This surely enables the film to impart a certain degree of the self-healing effects. However, reduction effects cannot be obtained for film damage caused when the film is in contact with, for example, a sharp-edged metal piece. In addition, since the resin to be included in the film is expensive, the method is problematic in both the economy and productivity.
In the methods of the type (2), the durability of the self-healing effects is improved in comparison to the case where water-soluble chromium acid is applied onto the steel-sheet surface to form the film. On the other hand, however, the water-solubility restricts the level of the self-healing effects that can be obtained. Even when the highest possible level of the self-healing effects is obtained in the above method, the level is equivalent to the level that can be achieved immediately after water-soluble chromium compound is included in the film. In addition, generally, films including the chromium hydrochloric acid tend to be discolored. The discoloration significantly reduces the value of products, particularly, products that are used without coating.
In a Znxe2x80x94Al-base-alloy-plated steel sheet, a sacrificial anticorrosion action and a passivation-film forming action work in a synergetic manner. This causes a high corrosion resistance to be imparted. Nevertheless, however, since the aluminum (Al) has inherent properties to form an active metal, when a passivation film is damaged, a blackened phenomenon easily occurs in a humid environment.
The following describes four methods proposed for inhibiting the blackening behavior of a Znxe2x80x94Al-base-alloy-plated steel sheet that contains 4 to 25 wt % Al:
(1) Method in which treatment is performed after plated is performed using solution that contains Ni ions and Co ions (according to JP-A-59-177381);
(2) Method in which a heat treatment is performed after skin-pass rolling (according to JP-A-55-131178);
(3) Method in which plated surfaces are cleaned using alkali water solution (according to JP-A-61-110777); and
(4) Method in which post-plating blasting is performed prior to a chromate treatment (JP-A-63-166974).
In practice, when these proposed methods are applied, an improvement effect can be recognized in a normal humid environment in regard to the antiblackening resistance of the Znxe2x80x94Al-base-alloy-plated steel sheet that contains 4 to 25 wt % Al. However, in many practical cases, during construction of building structures such as roofs, external walls, Znxe2x80x94Al-base-alloy-plated steel sheets that each contain 4 to 25 wt % Al are stored outdoors. Also, in many cases, the steel sheets are thus stored outdoors in a sheet state or in a state in which the materials as formed by, for example, roll-forming, are stacked. When the steel sheets are stored in this manner in a natural environment, the steel-sheet surfaces are easily wetted because of, for example, dewing. Thereby, the surfaces of the sheet materials are easily blackened in a couple of days.
In addition, any one of the above-described methods requires dedicated processing facilities. This arises problems in economy and productivity.
In addition, JP-B-1-53353, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies the xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a method of inhibiting the blackening behavior of an Alxe2x80x94Zn-base-alloy-plated steel sheet that contains 25 to 75 wt % Al. In this method, the treatment is performed using a treatment liquid made by mixing chromium acid and resin at a ratio that is at least a predetermined ratio. The treatment resultantly prevents chromium acid from directly reacting to the plating, and improves the antiblackening resistance. Moreover, JP-A-59-177381 and No. 63-65088 each disclose an antiblackening-resistance inhibiting method. In the method, pretreatment for chromate treatment is performed after plating by using Ni and Co.
According to the method in which the treatment is performed using the treatment liquid made by mixing chromium acid and resin at a ratio that is at least a predetermined ratio, antiblackening resistance can securely be obtained to a certain extent for the Znxe2x80x94Al-base-alloy-plated steel sheet that contains 25 to 75 wt % Al. However, complete resistance cannot be insured. In addition, since the resin is mixed with the chromium acid at a ratio that is higher or equal to a predetermined ratio, the service life of the treatment liquid is significantly shortened. To use the resin sufficient to withstand the oxidant effects of the chromium acid, the production cost increases. This makes the method to be disadvantageous in the cost.
In addition, since the pretreatment is performed using the metals such as Ni and Co, while the antiblackening resistance may be improved, use of the expensive metals increases the production cost.
It is an object of the present invention to provide a surface-treated steel sheet having a high corrosion resistance and a method for producing the same.
To achieve the object, first, the present invention provides a surface-treated steel sheet comprising a steel sheet, an Alxe2x80x94Zn-base alloy plating layer formed on the steel sheet, a chemical conversion film provided on the alloy plating layer, and a concentric layer of a Cr compound that is formed on the alloy plating layer of the chemical conversion film. The alloy plating layer contains Al in an amount of from 20 to 75%. The chemical conversion film is formed by applying a chemical treatment liquid containing principal components of an aqueous organic resin and chromic acid. The chemical conversion film has a weight ratio of resin/Cr in a range of from 20 to 200, and the coating weight of the Cr in a range of from 3 to 50 mg/m2 (as converted to metallic chromium). The concentric layer contains a Cr compound in a ratio of resin/Cr that is below the level that is 0.8 times a mean ratio of resin/Cr of the chemical conversion film.
Second, the present invention provides a surface-treated steel sheet comprising a steel sheet, a zinc-base plating layer formed on the steel sheet, a film that is formed on the zinc-base plating layer and that contains chromium in an amount of from 0.1 to 100 mg/m2 and calcium in an amount of from 0.1 to 200 mg/m2.
A method for producing the surface-treated steel sheet described above may comprise the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound, calcium or a compound of the calcium, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing. The treatment liquid contains hexavalent chromium ions in a range of from 0.1 to 50 g/l and calcium in a range of from 1 to 50 g/l.
In addition, a method for producing the surface-treated steel sheet may comprise the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound in which a chromium compound comprises a trivalent-chromium compound, and calcium or a compound of the calcium, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating the zinc-base-plated steel sheet at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing.
Third, the present invention provides a surface-treated steel sheet comprising a steel sheet; a zinc-base plating layer formed on the steel sheet; and a film that is formed on the zinc-base plating layer and that contains chromium and a compound containing phosphoric acid and at least one selected from a group of zinc and aluminum. The chromium is in an amount of from 0.1 to 100 mg/m2, and the compound is in an amount of from 0.1 to 100 mg/m2 as converted to phosphorus.
A method for producing the surface-treated steel sheet as described above may comprise the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound and one of phosphoric acid and salt thereof, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing. The treatment liquid contains hexavalent chromium ions in a range of from 0.1 to 50 g/l and phosphoric acid in a range of from 1 to 50 g/l.
In addition, a method for producing the surface-treated steel sheet as described above may be established to include the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound in which a chromium compound is composed of a trivalent-chromium compound, and one of phosphoric acid and salt thereof, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating the zinc-base-plated steel sheet at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing. The treatment liquid contains trivalent chromium ions in a range of from 0.1 to 50 g/l and phosphoric acid in a range of from 1 to 50 g/l.
Fourth, the present invention provides a surface-treated steel sheet comprising a steel sheet; a zinc-base plating layer formed on the steel sheet; and a film that is formed on the zinc-base plating layer and that contains chromium, calcium, and a compound containing phosphoric acid and at least one selected from a group of zinc and aluminum. The chromium is in a range of from 0.1 to 100 mg/m2, the calcium is in a range of from 1 to 200 mg/m2, and the compound is in a range of from 0.1 to 100 mg/m2 as converted to phosphorus.
A method for producing the surface-treated steel sheet as described above may comprise the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound, one of calcium and a compound thereof, and one of phosphoric acid and salt thereof, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing.
In addition, a method for producing the surface-treated steel sheet as described above may comprise the steps of (a) preparing a treatment liquid containing a water-soluble chromium compound in which a chromium compound is composed of a trivalent-chromium compound, calcium or a compound thereof, and one of phosphoric acid and salt thereof, (b) applying the treatment liquid onto a surface of a zinc-base-plated steel sheet, and (c) forming a film by heating at a highest-reachable sheet temperature in a range of from 60 to 300xc2x0 C. without performing rinsing.
Fifth, the present invention provides a surface-treated steel sheet comprising a steel sheet; a zinc-base plating layer that is formed on the steel sheet that contains 30 wt % zinc; and a film that is formed on the zinc-base plating layer and that contains an organic resin, Cr, Ca, and silica or a silica-group compound. The film is formed such that the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, the coating weight the Ca is in a range of from 0.001 to 0.2 in Ca/organic resin (weight ratio), and the coating weight of the silica or the silica-group compound is in a range of from 0.001 to 0.5 in SiO2/organic resin (weight ratio).
A method for producing the surface-treated steel sheet as described above includes the steps of:
(a) preparing an aqueous treatment liquid containing one of a water-soluble organic resin and a water-dispersible organic resin, one of water-soluble chromic acid and chromate, a Ca compound, and one of silica and a silica-group compound;
(b) applying the aqueous treatment liquid onto a surface of a zinc-base-plated steel sheet containing a zinc-base plating layer containing at least 30 wt % zinc; and
(c) drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C. without performing rinsing.
Sixth, the present invention provides a method for producing a surface-treated steel sheet, comprising the steps of:
applying chromate treatment onto a surface of a zinc-base-plated steel sheet containing at least 30 wt % zinc;
applying a treatment liquid containing an organic resin, a Ca compound, and one of silica and a silica-group compound; and
forming a film by drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C.
In the formed, the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, the coating weight of the Ca is in a range of from 0.001 to 0.2 in Ca/organic resin (weight ratio), and the coating weight of one of the silica and the silica-group compound is in a range of from 0.001 to 0.5 in SiO2/organic resin (weight ratio).
Seventh, the present invention provides a surface-treated steel sheet, comprising:
a steel sheet;
a zinc-base plating layer that is formed on the steel sheet that contains 30 wt % zinc; and
a film that is formed on the zinc-base plating layer and that contains an organic resin, Cr, Ca, and phosphoric acid or a phosphoric acid compound. The film is formed such that the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, the coating weight of the Ca is in a range of from 0.001 to 0.2 in Ca/organic resin (weight ratio), and the total coating weight of one of the phosphoric acid or the phosphoric acid compound is in a range of from 0.001 to 0.5 in PO4/organic resin (weight ratio).
A present invention provides a method for producing the surface-treated steel sheet as described above includes the steps of:
(a) preparing an aqueous treatment liquid containing one of a water-soluble organic resin and a water-dispersible organic resin, one of water-soluble chromic acid and chromate, a Ca compound, and at least one phosphoric acid compound selected from a group of zinc phosphate, aluminum phosphate, condensed zinc phosphate, and condensed aluminum phosphate;
(b) applying the aqueous treatment liquid onto a surface of a zinc-base-plated steel sheet containing a zinc-base plating layer containing at least 30 wt % zinc; and
(c) drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C. without performing rinsing.
Eighth, the present invention provides a method for producing a surface-treated steel sheet, comprising the steps of:
applying chromate treatment onto a surface of a zinc-base-plated steel sheet containing at least 30 wt % zinc;
applying a treatment liquid containing an organic resin, a Ca compound, and at least one phosphoric acid compound selected from a group of zinc phosphate, aluminum phosphate, condensed zinc phosphate, and condensed aluminum phosphate; and
forming a film by drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C.
The film is formed such that the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, the coating weight of the Ca is in a range of from 0.001 to 0.2 in Ca/organic resin (weight ratio), and the total coating weight of the phosphoric acid compound(s) is in a range of from 0.001 to 0.5 in PO4/organic resin (weight ratio).
Ninth, the present invention provides a surface-treated steel sheet comprising a steel sheet; a zinc-base plating layer that is formed on the steel sheet that contains 30 wt % zinc; and a film that is formed on the zinc-base plating layer and that contains an organic resin, Cr, and a complex compound containing Caxe2x80x94PO4xe2x80x94SiO2 as a principal component. The film satisfies conditions in which the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, a weight ratio of (Ca+SiO2+PO4)/organic resin is in a range of from 0.01 to 0.5, and a weight ratio of (Ca+SiO2)/PO4 is in a range of from 0.05 to 0.8.
A method for producing the surface-treated steel sheet as described above includes the steps of:
(a) preparing an aqueous treatment liquid containing one of a water-soluble organic resin and a water-dispersible organic resin, one of water-soluble chromic acid and chromate, and a complex compound containing Caxe2x80x94PO4xe2x80x94SiO2 as a principal component;
(b) applying the aqueous treatment liquid onto a surface of a zinc-base-plated steel sheet containing a zinc-base plating layer containing at least 30 wt % zinc; and
(c) drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C.
Tenth, the present invention provides a production method for a surface-treated steel sheet, including the steps of:
applying chromate treatment onto a surface of a zinc-base-plated steel sheet containing at least 30 wt % zinc;
applying a treatment liquid containing an organic resin and a complex compound containing Caxe2x80x94PO4xe2x80x94SiO2 as a principal component; and
forming a film by drying the applied treatment liquid at a sheet temperature in a range of from 60 to 250xc2x0 C.
The film is formed such that the coating weight of the organic resin is in a range of from 50 to 5,000 mg/m2, the coating weight of the Cr is in a range of from 1 to 100 mg/m2, a weight ratio of (Ca+SiO2+PO4)/organic resin is in a range of from 0.01 to 0.5, and a weight ratio of (Ca+SiO2)/PO4 is in a range of from 0.05 to 0.8.